Overpayment due to FEX movement

Occasionally we receive a bill from a supplier in US dollars, which is input to Clearbooks in dollars and a £ amount is calculated on that day's exchange rate.

When we pay off that bill, we instruct the bank to transfer the exact amount in dollars but when I come to reconcile this in the bank statement, it obviously shows that the £ amount going out of the bank is different, because that day's exchange rate is different to that of the bill date.

If our payment going out is short, I understand that I can write off the unpaid balance of the bill using the FX tab at the bottom of the bill page (next to the total).

However, if our payment going out is short, I have to do something with the overpaid amount, even if it's just £2. How should I do this?

I have been using the Transaction tab on the bank reconciliation to assign the full amount of the bill to the supplier, but then with the £2 over, I've changed the dropdown to, say, Barclays current (the account it's being paid out from) and choosing Forex movement. This comes up with a warning to say that bank accounts aren't usually associated with Forex movement. But how else should I allocate the overpayment?

Sometimes this comes up as a PAY000 transaction but more recently it's been created PUR000 transactions which appear on our purchase ledger, even though I don't want them to.

Please help! Thanks Lisa